The present invention relates to wireless communication technology, and, more particularly, to antenna interface circuits and electronic devices incorporating the same.
Mobile terminals are widely used for voice and/or data communications. It is often desirable for a mobile terminal to transmit and receive over multiple frequency bands, for example, to provide both PCS and GSM capabilities. It may also be desirable to provide mobile terminals that operate over four radio frequency (RF) bands. For example, it may be desirable to provide a wireless terminal that can operate over the GSM850 band that is used in the United States (also referred to herein as GSM), the EGSM900 band that is used in Europe (also referred to herein as EGSM), the DCS1800 band that is used in Europe (also referred to herein as DCS) and the PCS1900 band that is used in the United States (also referred to herein as PCS). The transmit (TX) and receive (RX) frequencies of these bands are shown in Table 1:
TABLE 1Frequency RangeBand 824 Mhz-849 MhzGSM850 TX Band: US 869 Mhz-894 MhzGSM850 RX Band: US 880 Mhz-915 MhzEGSM900 TX Band: Europe 925 Mhz-960 MhzEGSM900 RX Band: Europe1710 Mhz-1785 MhzDCS1800 TX Band: Europe1805 Mhz-1880 MhzDCS1800 RX Band: Europe1850 Mhz-1910 MhzPCS1900 TX Band: US1930 Mhz-1990 MhzPCS1900 RX Band: US
It may also be desirable for a mobile terminal to operate over multiple frequency bands used in third generation (3G) wireless technologies, such as the Universal Mobile Telephone System (UMTS). For example, Table 1 lists the transmit and receive frequency ranges for four bands used in UMTS networks:
TABLE 2Frequency RangeBand1920 MHz-1980 MHzBand 1 transmit2110 Mhz-2170 MhzBand 1 receive1850 Mhz-1910 MhzBand 2 transmit1930 Mhz-1990 MhzBand 2 receive1710 Mhz-1785 MhzBand 3 transmit1805 Mhz-1880 MhzBand 3 receive 824 Mhz-849 MhzBand 5 transmit 869 Mhz-894 MhzBand 5 receiveA quad band antenna interface module may be used to interface between an antenna port and the RF circuitry for the four bands. The design of the interface between the antenna port and the RF circuitry may play a role in achieving the published customer requirements and in providing a desired Over-The-Air (OTA) performance. Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) are two figures of merit that define OTA performance. Both of these parameters are typically defined for the frequency band of interest. For a quad band device, the OTA performance is typically defined for eight frequency sub-bands.
In many conventional mobile terminals, an antenna feed-point port is the location where the antenna connects to the RF circuitry. FIGS. 1A and 1B are Smith charts that illustrate the antenna feed-point impedance for a conventional mobile terminal for the 824 MHz-960 MHz frequency band and the 1.7 GHz-1.990 GHz frequency bands, respectively. Many mobile terminals include RF circuitry that is designed for a 50 ohm nominal system impedance. As can be seen from FIGS. 1A and 1B, the low-band impedance locus and the high-band impedance locus differ significantly. Impedance matching may, therefore, be used to improve OTA performance.
FIG. 2 illustrates a conventional impedance matching circuit 200 that may be used between a radio interface port and an antenna feed-point port to connect an antenna 210 to RF circuitry in a mobile terminal. The impedance matching circuit 200 includes a plurality of impedance elements Z1 through Z8 along with a transmission line 205 that are configured as shown. As illustrated in FIGS. 1A and 1B above, the impedance that the impedance matching circuit 200 is designed to match varies based on frequency. As a result, the design of the impedance matching circuit 200 is typically based on a compromise to achieve an acceptable level of performance across multiple frequency bands, but may not provide the best possible performance for any specific frequency band. Such a design can be complex and potentially costly in terms of the amount of time involved to arrive at a design that provides acceptable performance. Moreover, the matching circuit 200 may consume a relatively large amount of area on a circuit board.
FIG. 3 illustrates a conventional impedance matching circuit 300 that may be used between the antenna feed-point ground for an antenna 305 and the radio circuit ground. As shown in FIG. 3, the impedance matching circuit 300 includes a switch 310 that connects either a capacitor C1 or an inductor L1 between the antenna feed-point ground and the radio circuit ground. Unfortunately, the switch 310 is typically implemented using a transistor circuit, which can generate undesired RF harmonics resulting in increased Radiated Spurious Emissions (RSE). While the switch 310 can be implemented using a diode design, this may result in increased current usage and, as a result, may lessen the battery life in the mobile terminal.